1. Field of the Invention
This invention relates to the manufacture of rotary valves such as are used in hydraulic power steering gears for vehicles and in particular valves as disclosed in International Patent Application PCT/AU93/00015. That application describes a low noise rotary valve having a unique linear boost characteristic.
2. Description of the Related Art
In general, such rotary valves include an input-shaft which incorporates in its outer periphery a plurality of blind-ended, axially extending grooves separated by lands. Journalled on the input-shaft is a sleeve having in its bore an array of axially extending blind-ended slots matching the grooves in the input-shaft, but in underlap relationship thereto, the slots of one being wider than the lands of the other so defining a set of axially extending orifices which open and close when relative rotation occurs between the input-shaft and the sleeve from a centred position, the magnitude of such rotation henceforth referred to as the valve operating angle. The edges of the input-shaft grooves are contoured so as to provide a specific orifice configuration and are referred to as the metering edge contours. These orifices are ported as a network such that they form sets of hydraulic Wheatstone bridges which act in parallel to communicate oil between the grooves in the input-shaft and the slots in the sleeve, and hence between an engine driven oil pump, and right-hand and left-hand hydraulic assist cylinder chambers incorporated in the steering gear. The input-shaft and sleeve are biased towards the centred position by a torsion bar spring as is well known in the art. The relationship between the level of power assistance generated in the valve, as a function of input torque, is known as the boost characteristic, and is largely determined by the metering edge contours.
It is convenient to refer to the metering edge contours as controlling three zones of the boost characteristic, the high pressure contour associated with the parking zone, an intermediate contour associated with the cornering zone, and a central zone associated with straight ahead driving where normally minimum boost is desired. For many vehicles it is becoming increasingly accepted that in the critical cornering zone, a truly linear relationship should exist between input torque applied by the driver and the level of power assistance, that is, a linear boost characteristic.
At the same time, increasing emphasis is placed on achieving a very low noise level, particularly when the valve is operating at high pressures such as in parking. Typically this is achieved by having each high pressure contour associated with the parking zone comprise a shallow chamfer which, in section, is inclined at a slope between about 4.degree. and 8.degree. at the junction between the chamfer and the periphery of the input-shaft. By this means, the overall flow at high pressures is divided equally into a series of thin sheets or jets between the several orifices which are closing simultaneously. Such thin sheet jets have been shown to be less likely to produce cavitation and associated valve hiss. At larger chamfer angles than about 8.degree., flow separation between the oil flow and adjacent periphery of the input-shaft may occur, also leading to cavitation. The shallow chamfer typically subtends an angle of about one degree at the input-shaft axis and thereafter extends towards the input-shaft groove edge as an intermediate contour in the form of a steeper chamfer or, preferably, in the form of a curved chamfer or scroll of decreasing radius. Such a latter scroll concept is earlier taught in EPO Patent 0196172 and provides a means of achieving a linear boost relationship in the cornering zone for a low noise valve.
The particular feature of International Application PCT/AU93/00015 is the addition of a longitudinal trough lying within some or all of the cornering zone chamfers, this trough serving to extend the linear boost region which otherwise ceases to be linear about halfway through the desired pressure range due to the changing orifice characteristic resulting from the increasing viscous drag of the oil as the jet thickness decreases towards the parking zone chamfer.
In a typical example the trough has a bottom surface substantially parallel and concentric with the outside diameter of the input-shaft and terminates circumferentially in an abrupt axially extending escarpment leading to the parking zone chamfer on the one side, and on the other side opens to the cornering zone chamfer or side of the longitudinal groove.
The trough may be substantially axially shorter than the parking zone chamfer and adjacent cornering zone chamfer; also the trough may be incorporated in all or only in some of the metering edge contours. These factors allow relatively deep troughs to be used and hence the input-shafts to be manufactured using the roll imprinting process. However the use of such axially shorter troughs means that it is not possible to achieve a precisely uniform distribution of oil along the full length of all chamfers when the valve is operating in the parking zone, as oil reaches the start of each of these chamfers at a substantially higher pressure for axial positions where the trough is present than for other axial positions where no trough is present. This may cause increased levels of cavitation. Nevertheless for designs of valves where a low oil flow is used, this method still provides an efficient means of achieving acceptably low noise levels. For other designs requiring a relatively high oil flow and/or very low noise levels, it is preferred that the troughs extend along all metering edge contours most of their length, in which case they may be required to be as shallow as 20 um to 30 um. Because the roll imprinting process is carried out prior to input-shaft hardening and finish grinding of the outside diameter, such roll imprinted troughs are inevitably subject to random depth variations due to component distortion which could be tolerated if the troughs were relatively deep, but cannot be tolerated if they are only 20 um to 30 um deep.
In such cases it is preferred that the troughs be produced, not by roll imprinting, but by grinding after the input-shaft is hardened and finish ground on the outside diameter. It is further preferred that this trough grinding process occurs after grinding of the remaining chamfer or chamfers of the metering edge contour, however both grinding processes can be carried out in a single operation without removing the input-shaft from the machine. This can achieve the very high precision needed in angularly positioning the troughs relative to the chamfers, whereas to re-establish this angular relationship accurately if a separate later operation were used would be very difficult due to the shallow angle of these chamfers.
However, because of the re-entrant angle formed by the bottom surface and the escarpment of the trough, such troughs cannot be ground by conventional chamfer grinding machines, such as disclosed in International Patent Application PCT/AU91/00494, where the axis of the grinding wheel is maintained substantially parallel to the axis of the input-shaft. A machine and process capable of grinding such a trough is described in the present specification, as is a trough having particularly desirable characteristics.